Crucial to understanding the physical microenvironment of articular cartilage and to knowledge of its functional capacity to distribute loads is the determination of the magnitude and distribution of the load across the joint. Although the resultant of forces across the hip joint has been estimated by several different means, the distribution and the magnitude at specific sites has never been determined. Only by obtaining this information can the function of cartilage be assessed. An instrumented femoral head replacement prosthesis has been designed and the components tested which will sense pressure normal to the surface of the prostheshs at 14 sites in the weight bearing area of the hip joint in vivo. When inserted into a patient requiring femoral head replacement, direct data on weight distribution across the hip joint will be telemetered out. Extensive in vivo physiologic studies of the nature and function of cartilage will be possible. Using the instrumented prosthesis in conjunction with a hip simulator, extensive in vitro studies of the human hip socket will be done investigating structural changes and the effect of arthritis on the pressure profile in the hip.